1. Field of the Invention
This invention relates to a method for measuring magnetic or electric fields within a sample material and, more particularly, to a method using a scanning force microscope to track the topography of the surface so that such fields occurring close to the material surface are accurately measured at a small, constant distance from the surface.
2. Background Information
The measurement of a magnetic or electric field occurring within a sample material, from a measurement point traversing the surface of the material at a very close spacing beyond this surface, has been a subject of an ongoing effort by scanning probe microscopy researchers for several years. A measurement process of this type requires that a probe sensitive to magnetic or electric fields must be moved in a direction perpendicular to the sample surface during the traversing, or scanning process, so that the probe tracks the topography of the sample surface without contacting it. Moving the probe in this way is both important and difficult when the sample surface is quite rough, as is the surface of a number of materials for which this type of measurement can provide significant information, such as the surfaces of magnetic data storage media.
The scanning force microscope provides an accurate method for moving a probe along a surface in very close proximity thereto. A probe having a very sharp tip is moved along the sample surface being examined by means of a lateral actuator. The probe is mounted to a distal end of a cantilever, the proximal end of which is attached to a vertical actuator, which moves the probe tip into and out of engagement with the sample surface. Vibration in this vertical direction is applied to the distal end of the cantilever through the vertical actuator at a frequency close to the resonant frequency of the cantilever. The vibration of the probe tip at this frequency is measured. As topographical features of the sample surface increase the engagement of this surface with the probe tip, the probe tip vibration is decreased. As this engagement is decreased, the probe tip vibration increases up to a point at which the probe is freely vibrating out of contact with the sample surface. A feedback signal is generated as a difference between a signal representing probe tip vibration and a setpoint signal representing a level of vibrations occurring with the operational level of engagement desired between the probe tip and the sample surface. This feedback signal is used within a servomechanism loop including the vertical actuator to maintain the engagement at this operational level during lateral scanning.
However, when a single probe is used to track the surface topography, with a method such as that of the scanning force microscope, and simultaneously to track magnetic or electric fields, the signals produced by changes in topography tend to become mixed with the signals caused by these fields, so that accurate information cannot be recovered. What is needed is a way for separating the measurement of topography from the measurement of a field, while moving the probe in response to topographical variations during field measurements.
3. Description of the Prior Art
U.S. Pat. No. 4,724,318 describes an atomic force microscope, in which a sharp point is brought so close to the surface of a sample to be investigated that the forces occurring between the atoms as the apex of the point and those at the surface cause a spring-like cantilever to deflect. The cantilever forms one electrode of a tunneling microscope, the other electrode being a sharp tip. The deflection of the cantilever provokes a variation of the tunnel current, and that variation is used to generate a correction signal which can be employed to control the distance between the point and the sample, in order, for example, the force between them constant as the point is scanned across the surface of the sample by means of an xyz-drive, with the sample being driven in a raster scan in the xy-plane. In certain modes of operation, either the sample or the cantilever may be excited to oscillate in the z-direction. If the oscillation is at the resonance frequency of the cantilever, the resolution is enhanced. Using this method, a topographical image of a sample surface having a resolution better than 100 nanometers may be obtained by employing the following steps: A sharp point which is fixed to one end of a spring-like cantilever is brought so close to the surface of the sample to be inspected that the forces occurring between the point and the sample surface are larger than 10.sup.-20 Newton, so that the resulting force deflects the cantilever. The deflection of the cantilever is detected by means of a tunnel tip disposed adjacent the cantilever. The tunnel current then flowing across the gap between the cantilever and tunnel tip is maintained at a constant value by using any detected variations of the tunnel current to generate a corrections signal. The correction signal is used, among other things, to maintain the point-to-sample distance constant.
Several methods have been developed for separating the measurement of surface topography from the simultaneous measurement of magnetic and electric fields. For example, D. Ruger, et al., writing in the Journal of Applied Physics, Vol. 68(3)., page 1169 in 1990, describe the measurement of magnetic forces by applying an additional direct current bias of 0 to 10 volts between the probe and the sample. This electrical bias provides an attractive electrostatic force gradient which is only a function of the distance between the probe and the sample surface. This attractive electrostatic force combines with the magnetic forces to be measured, which vary along the sample surface, to form an overall attractive force gradient that increases in magnitude as the probe approaches the surface. This additional attractive force assures that the overall force remains attractive, so that the servo loop keeping the probe tracking above the sample surface is stabilized, despite the fact that magnetic forces may be either attractive or repulsive.
However, the contours of the constant-force gradient measured in this way do not necessarily reflect only a level of magnetic contrast, due to the dependence of the non-magnetic forces on the distance between the probe tip and the sample surface. That is, if the force gradient of the non-magnetic force is a nonlinear function which is comparable in magnitude to the measured magnetic force gradient, then the resulting vertical-axis response of the apparatus to the magnetic force gradient is also nonlinear. Furthermore, when this method is used on a rough surface, the distance between the probe tip and the surface must be relatively large to avoid the contamination of the magnetic-force gradient map with topographical data. When this distance is large, the spatial resolution and signal-to-noise ratio of the magnetic force measurements are decreased.
Another method for separating the simultaneously-occurring signals caused by responses to variations in topography and magnetic fields is the use of modulation technology, as described by Martin and Wickramasinghe, in Applied Physics Letters, vol. 50, page 1455, in 1987, and by Schonenberger et al. in the Journal of Applied Physics, Vol. 67, page 7278, in 1990. With this technique, for example, adding an AC modulation signal to the applied DC bias causes a second-harmonic oscillation of the cantilever. The amplitude of this oscillation is used to drive a feedback loop adjusting the separation between the probe tip and the sample surface. While the vertical (Z-direction) movement of a piezoelectric actuator required to hold the tip vibration amplitude at a pre-determined value is used, in the manner of a conventional scanning force microscope, to provide an indication of the surface topography, the DC force is measured by detecting quasistatic deflections of the cantilever, as indicated by variations in the difference between the deflection of the probe tip and the movement of the actuator. A disadvantage of this method lies in the difficulty of measuring these quasistatic deflections in a manner providing an accurate depiction of the underlying electric or magnetic field. Such deflections are not easily measured with the accuracy available through the use of AC methods depending on changes in the amplitude of vibrations occurring at a frequency near the resonate frequency of the cantilever.
Another method for measuring magnetic fields, or other non-topographical features of a sample, is described in U.S. Pat. No. 5,418,363. This method employs a first pass of the probe along a scan line on the surface of the sample to determine its topography by conventional means, such as by the method of atomic force microscopy. During this first pass, the topographical data is stored. In a second pass, the sample surface is scanned along the same line, with this stored topographical data being used to ensure that the probe is at all times displaced from the sample surface by a pre-determined offset distance.
In an embodiment of such apparatus configured to measure magnetic fields, a probe incorporates a magnetic tip, and the first scan is provided using the method of an atomic force microscope. During the second scan, the magnetic tip is maintained above the surface of the sample to allow the measurement of magnetic forces in the sample. During this second scan, the deflection of the cantilever, may be measured directly by a detector, or the cantilever may be vibrated near resonance, with the resulting amplitude or phase of vibration being measured by a detector.
An example of measurements made with such apparatus is given by Y. E. Strausser et al., in American Laboratory, May, 1994, is a measurement of the topography and magnetic force gradients of the surface of a hardfile disk. In this example, the topography had variations of 120 nm peak-to-peak. The tip followed this topography, but at a displacement 100 nm above the surface.
In an embodiment of such apparatus configured to measure electric fields, the apparatus includes a conducting tip, with the first scan being performed as either an atomic force microscope or a scanning tunneling microscope. During the second scan, a voltage source provides for a voltage differential between the tip and the sample. In this way, an electric field is developed between the tip and the sample, indicating, for example, if the sample is an integrated circuit, the presence of various circuit elements below the surface.
What is needed is a method eliminating the need to move the center of vibrations away from the sample surface during the measurement of the magnetic or electric field, so that the spacial resolution and signal-to-noise ratio of the field measurements is not degraded. Also, to speed the measurement process, what is needed is a way to measure both topographical features and the magnetic or electric field during a single pass.
The use of a vibrating probe to measure gradients within a force field has been described by R. Wiesendanger in Scanning Probe Microscopy and Spectroscopy--Methods and Applications, Cambridge University Press, 1994, on pages 241-243. In such a field, the effective spring constant is given by: ##EQU1## In the above equation, c is the spring rate of the cantilever in the absence of a force field, and c.sub.eff is the effective spring rate of the cantilever in the presence of the force field. In an attractive force field, with the probe tip being attracted to the surface, the cantilever is effectively softened. In a repulsive force field, with the probe tip being repelled by the surface, the cantilever is effectively stiffened.
The change in the resonant frequency of vibrations of the cantilever/mass system is given by: ##EQU2## In the above equation, m is an effective mass, and .omega..sub.0 is the resonant frequency of the system in the absence of a force gradient.
Various methods for making a probe tip suitable for the measurement of magnetic forces under the conditions of non-contact force microscopy are described by R. Wiesendanger, ibid, pages 253-256. A first example of a magnetic force sensor is a cantilever with an integrated tip formed by electrochemical etching of a thin nickel, iron, or cobalt wire. After the wire is etched, it is bent at its distal end to form a tip. Alternately, a non-magnetic tip, composed of a material such as tungsten, after surface preparation by electrochemical etching, is coated with a thin layer, typically 50 nm, of ferromagnetic material, either by sputtering or by galvanic deposition. This layer is magnetized after deposition. A magnetic tip coating technique has also been applied to microfabricated silicon cantilevers, which may be built to provide specific properties based on a choice of coating material and thickness.